Many conventional wireless communication systems use heterodyning techniques for transmitting and receiving signals. Heterodyning techniques involve multiplying or mixing an information-carrying signal (referred to as a message signal) with a purely sinusoidal signal (referred to as a carrier) to produce an output signal having a frequency translated by an amount equal to the frequency of the carrier (referred to as the carrier frequency). For example, a heterodyne transceiver may be used to modulate a base-band message signal to a radio frequency (RF) and demodulate an RF-modulated signal to baseband. A heterodyne transceiver may apply one or more filters and amplifiers to an RF-modulated signal before either transmitting the signal through air or demodulating the signal to baseband. A baseband signal may be formed using orthogonal frequency-division multiplexing (OFDM) in which multiple message signals are modulated to orthogonal frequency bands and added together. The message signals may be modulated using a variety of modulation techniques, examples of which include amplitude modulation (AM) and frequency modulation (FM). OFDM allows multiple message signals to be modulated to RF and transmitted simultaneously.
A carrier with a stable frequency is produced by a local oscillator. A local oscillator uses negative feedback to adjust the frequency of the carrier. A local oscillator may include analog circuitry, digital circuitry, or a combination of analog and digital circuitry. A local oscillator may include a direct digital synthesizer (DDS), which is a digital component that generates a frequency- and phase-tunable sinusoidal signal that is referenced to a fixed-frequency clock. In some DDS devices, the frequency and phase of the output signal is programmable and may be adjusted while the DDS is in operation. Most DDS devices include a phase accumulator, a waveform lookup table, and a digital-to-analog converter (DAC). The DDS may include phase and frequency registers or RAM for generating various modulation schemes. The DDS may also include a comparator for generating a clock signal from its sinusoidal output.